I have an underground p trap for my tub, how to prevent it from freezing?

Hi guys, I have a condo that is built on a slab and am currently installing the plumbing to a new tub. There is no access to my p trap once a tub is in place due to the house being on a slab. Below the slab is about 13 feet of open space, so the p-trap will just be exposed to the air that is enclosed under my home. This air is not temperature controlled and is basically the temperature it is outside except that it is surrounded by cement walls. I am worried about the water freezing in the p trap during the cold winter months (temperatures can reach 0 degrees F here in the midwest at times), or the frigid temperatures cracking my pvc cement bonds. I don't know if this is possible, but I was concerned about it.

My questions are: Will the water freeze in my schedule 40 pvc p trap if the tub is not used more than once a month, even though my heat is on inside the condo? Do I need to put pipe insulation around the p trap or is this unnecessary? Do I need to actually go down there and insulate the cement walls to keep the temperature warmer in there? Or do I need to buy some heating device, like heat tape?

Thanks, just trying to figure out how my plumbing will be okay if there is no insulation down there since it can be so cold. I appreciate the help

When you refer to your condo being on a slab what material are you talking about? Is this a new location for plumbing, was there a tub there before? If so how was it weather proofed before? You say you have 13' of space below, is this soil or is this where the slab is?

Hey, thanks for the reply. When I say slab, I mean the house was built on a layed cement floor. It is not on a wooden frame, it is on a cement and metal frame. So when you pull up the floor, there is just a slab of cement. There wasn't any insulation there before, but there also was someone using that tub regularly. In the future, it will be used much less because my roommate may move out and that is when I will be concerned. I have posted pics of what it looks like under the p trap so you can see what i mean. Thanks again for your reply, and let me know if you think I should do anything to protect the trap

Not that I know of, I was more worried about it freezing when it wasn't in use when my roommate moves out. But I am sure I am being overly cautious/anal and it should be fine. I may just put some pipe insulation on the trap

Not that I know of, I was more worried about it freezing when it wasn't in use when my roommate moves out. But I am sure I am being overly cautious/anal and it should be fine. I may just put some pipe insulation on the trap

Low voltage heating wires the stuff used on roofs to melt the snow and ice maybe something to consider.